Rethinking the Source of Earth’s Water

Earth, with its blue hue visible from space, is known for its abundant water – predominately locked in oceans – that may have come from an extraterrestrial source. New research indicates that the source of Earth’s water isn’t from ice-rich comets, but instead from water-bearing asteroids.

Looking at the ratio of hydrogen to deuterium, a heavy isotope of hydrogen, in frozen water, scientists can get a pretty good idea of the distance the water formed in the solar system. Comets and asteroids farther from the Sun have a higher deuterium content than ice formed closer to the Sun. Scientists, led by the Carnegie Institution for Science’s Conel Alexander, compared water from comets and from carbonaceous chondrites. What they found challenges current models in how the solar system formed.

Primeval Earth was a hot and dry place. Any water that may have formed with Earth was boiled away from the scorching crust. Ultraviolet light from the newly formed Sun stripped hydrogen atoms from the water molecules leaving no rain to fall back on the surface. Scientists believe that both comets and carbonaceous asteroids formed beyond the orbit of Jupiter, perhaps at the very fringes of the solar system, then moved inward bringing both water and organic material to Earth. If this were true, Alexander and his colleagues suggest that ice found in comets and the remnants of ice preserved in carbonaceous chondrites in the form of clays would have similar isotopic composition.

After studying 85 carbonaceous chondrites, supplied by Johnson Space Center and the Meteorite Working Group, they show in a paper released today by Science Express that they likely did not form in the same regions of the solar system as comets because they have much lower deuterium content. They formed closer to the Sun, perhaps in the asteroid belt between Mars and Jupiter. And its that material that rained on early Earth to create the wet planet we know today.

“Our results provide important new constraints for the origin of volatiles in the inner solar system, including the Earth,” Alexander said. “And they have important implications for the current models of the formation and orbital evolution of the planets and smaller objects in our solar system.”

Image caption: Artist impression of an asteroid impact on early Earth (credit: NASA)

I found that press release somewhat curious though. What I know of, comets have been considered a major water source for inner planets for sime time. Earth D/H ratio precludes anything more than ~ 10 % contribution from comets IIRC.

Earth has ~ 0.05 % by mass water, ~ 0.023 % as ocean and about as much in the crust and mantle.

But recent finds predict that Moon and Mars has about the same water content as Earth. Naively it is difficult to predict that from impactors. For example, the Moon which originated before the late heave bombardment would have had ~ 20 times less late accreation than the gravitationally larger target Earth.

Perhaps most water was present at accreation. One mechanism would have been physisorbtion to dust which can retain that much water up to ~ 500 K in the disk. IIRC the disk close to the protostar would have reached ~ 1000 K as witnessed by some found grains, while the ice line of ~ 0 K lies should have formed between Mars and Jupiter I think. (Ceres is believed to have lots of ice; certainly Europa has.)

I found the press release curious though. Earth and comet D/H ratios has precluded comets from contributing more than ~ 10 % of Earth water since the comet encounter missions, what I know of.

Carbonaceous chondrites (CC) may have supplied a dominant part of Earth water.

The late accretion, continuing after the Earth core formation stopped, may have supplied ~ 0.3 – 0.8 % of mantle mass. [“182W Evidence for Long-Term Preservation of Early Mantle Differentiation Products”, Touboul et al, Science, 2012.] Since Earth’s core is ~ 30 % by mass and carbonecous chondrites are ~ 10-20 % by mass the water supplied may have been ~ 0.3*(0.03*0.1 – 0.08*0.2) or ~ 0.01 – 0.05 % by mass.

The Earth is ~ 0.05 % water by mass, ~ 0.023 % in the oceans and about as much in crust and mantle. Hence CC could predict a dominant part of the observed water.

But recent results predict that Earth, Moon and Mars had early about the same amount of water in their mantles. Naively it is hard to predict impactors supplying that. For example, Earth is ~ 20 times as large a target for impactors than the Moon due to gravitational dominance.

Maybe the dominant part of mantle water was supplied at aggregation. Physiosorption of water to dust retain that much water up to ~ 500 K. The protoplanetary disk closest to the Sun reached ~ 1000 K as witnessed by some found grains IIRC. And the ice line at ~ 270 K formed somewhere between Mars and Jupiter what I know. (Ceres is believed to be ice covered; certainly Europa is.)

[Note: Something funny with the page script. This comment doesn’t post in my browser, after 2 tries, but are visible to me as the page loads. C&P from the dynamically loaded page worked though, so belatedly my comment is now visible to me.]

Another good catch.

I found that press release somewhat curious though. What I know of, comets have been considered a major water source for inner planets for some time. Earth D/H ratio precludes anything more than ~ 10 % contribution from comets IIRC.

Chondrites may have provided the dominant part of Earth water.

The continued late accretion, after the Earth core formation stopped, could have added ~ 0.3 – 0.8 percent of mantle mass. Since the core is ~ 30 percent by mass and carbonaceous chondrites have ~ 10-20 percent water by mass, the water supplied would be ~ 0.3*(0.1*0.003 – 0.2*0.008) or ~ 0.01 – 0.05 percent by mass.

Earth has ~ 0.05 % by mass water, ~ 0.023 % as ocean and about as much in the crust and mantle.

But recent finds predict that Moon and Mars has about the same water content as Earth. Naively it is difficult to predict that from impactors. For example, the Moon which originated before the late heave bombardment would have had ~ 20 times less late accretion than the gravitationally larger target Earth.

Perhaps most water was present at accretion. One mechanism would have been physisorption to dust which can retain that much water up to ~ 500 K in the disk. IIRC the disk close to the protostar would have reached ~ 1000 K as witnessed by some found grains, while the ice line of ~ 0 K lies should have formed between Mars and Jupiter I think. (Ceres is believed to have lots of ice; certainly Europa has.)

[Note: Something funny with the page script. This comment doesn’t post in FF or IE, but are visible to me as the page loads. I managed to C&P from the dynamically loaded page, and are now posting segments to see where the offending test is. -> seems to have been some invisible command character which I removed by happenstance C&P.]

Another good catch.

I found that press release somewhat curious though. What I know of, comets have been considered a major water source for inner planets for some time. Earth D/H ratio precludes anything more than ~ 10 % contribution from comets IIRC.

Chondrites may have provided the dominant part of Earth water.

The continued late accretion, after the Earth core formation stopped, could have added ~ 0.3 – 0.8 % of mantle mass. Since the core is ~ 30 percent by mass and carbonaceous chondrites have ~ 10-20 % water by mass, the water supplied would be ~ 0.3*(0.1*0.003 – 0.2*0.008) or ~ 0.01 – 0.05 % by mass.

Earth has ~ 0.05 % by mass water, ~ 0.023 % as ocean and about as much in the crust and mantle.

But recent finds predict that Moon and Mars has about the same water content as Earth. Naively it is difficult to predict that from impactors. For example, the Moon which originated before the late heave bombardment would have had ~ 20 times less late accretion than the gravitationally larger target Earth.

Perhaps most water was present at accretion. One mechanism would have been physisorption to dust which can retain that much water up to ~ 500 K in the disk. IIRC the disk close to the protostar would have reached ~ 1000 K as witnessed by some found grains, while the ice line of ~ 0 K lies should have formed between Mars and Jupiter I think. (Ceres is believed to have lots of ice; certainly Europa has.)

That found that Osmium, Oxygen, Deuterium isotopes and the Mg/Si and Al/Si ratios show that the upper mantle was unlikely to have come from any known meteoritic material (chondrite or achondrite) and comets could have given just 15-50% of earth water.